Many industries discharge warm and hot process and waste gases to atmosphere such that the heat energy is lost. This can also result in a contribution to atmospheric warming. Warm liquids such as waste waters can also be released to the environment. It would be desirable if some of this wasted heat energy could be captured and utilised in that industry or elsewhere.
Attempts have been made to develop systems for capturing waste heat. For example, U.S. Pat. No. 5,522,228 discloses apparatus for the production of cold by the adsorption and desorption of carbon dioxide. WO 2005/073644 (to the present applicant) discloses a method, apparatus and system for transferring heat which can be embodied as an adsorption chiller that employs a simple modular design principle.
However, the most significant hurdle to commercialisation currently being experienced with waste heat-driven chillers relates to the achievement of a satisfactory coefficient of performance (COP). Whilst the adsorption chiller of WO 2005/073644 may have a theoretically estimated maximum COP of close to 0.6, in practice a maximum COP of about 0.4 has been achieved.
Known gas fired, solar and waste heat driven coolers can be compression cycle, LiBr-water absorption or silica gel-water adsorption based. The compression cycle chillers use hydro-fluoro carbons (HFCs) as a working fluid and can achieve a maximum overall COP of about 1, based on 30% efficiency of the prime mover which drives the compressor of the compression cycle which has a COP of around 3.5. Compression cycles have many moving parts, especially with the use of a gas engine as prime mover. The maintenance requirement of these systems is typically higher as compared to electrically driven systems. Also HFC is a green house gas which must be phased out by 2020. On the other hand, known absorption and adsorption systems have a COP of up to 0.9 but have severe corrosion and maintenance issues. Both absorption and adsorption systems use cooling towers to supply cold water for condensing the refrigerant vapours.
By employing in a single stage, lithium bromide, a COP of 0.4 has been practically possible and a higher COP of up to 1.5 possible with a so-called triple effect system. However, these latter systems require additional heat exchangers, higher regeneration temperatures and a complicated flow scheme. A triple effect system also has higher capital costs and maintenance requirements as compared to a single effect system.
The compression cycle has dominated direct gas fired refrigeration as well as the electrically driven refrigeration markets, purely because of its higher COP. High COP can therefore be considered as the main criteria for the commercial success of any alternative cooling system. A current benchmark COP for the compression cycle is 3.5, and no waste heat-driven coolers have to date been able to achieve anything approaching a COP of 3.5.
It is to be understood that references to prior art information herein do not constitute an admission that the information forms part of the common general knowledge of a person of ordinary skill in the art in Australia or in any other country.